Complementary metal-oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs). A CMOS image sensor typically comprises an array of picture elements (pixels), which utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photo-diode formed in a silicon substrate. As the photo-diode is exposed to light, an electrical charge is induced in the photo-diode. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. The electrons are converted into a voltage signal in the pixel and further transformed into a digital signal.
A CMOS image sensor, which may be referred as a CMOS sensor, may comprise a plurality of dielectric layers and interconnect layers formed on the substrate, connecting the photo diode in the substrate to peripheral circuitry. The side having the dielectric layers and interconnect layers is commonly referred to as a front side, while the side having the substrate is referred to as a backside. Depending on the light path difference, CMOS image sensors can be classified as front-side illuminated (FSI) image sensors and back-side illuminated (BSI) sensors.
In an FSI image sensor, light from the subject scene is incident on the front side of the CMOS image sensor, passes through dielectric layers and interconnect layers, and falls on the photo diode. In contrast, in a BSI image sensor, light is incident on the backside of the CMOS image sensor without the obstructions from the dielectric layers and interconnect layers. As a result, light can hit the photo diode through a direct path. Such a direct path helps to increase the number of photons converted into electrons, which makes the CMOS sensor more sensitive to the light source.
In order to improve quantum efficiency of BSI image sensors, the substrate of BSI image sensors may be thinned. In addition, through an ion implantation process, a thin P+ layer may be formed on the thinned substrate to further improve quantum efficiency. Subsequently, a laser annealing process may be performed to activate the implanted P+ ions as well as repair crystal defects caused by the ion implantation process. Such a laser annealing process may cause dark mode image stripe patterns due to laser scanning boundary effects on the image sensor. To avoid such dark mode image stripe patterns, a thin oxide layer may grow to protect the silicon surface. Such a sequence of process on the backside of the substrate is long and costly. Methods for reducing the processing sequence for the BSI sensors while improving quantum efficiency are of interest.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.